If the screen goes black on a computer/laptop, this generally means that the computer has gone to sleep. The computer has to be awake/active in order for the mandalas to work. To keep the computer awake, we recommend using a free program like Don't Sleep, which you can easily find on Google.
Now, if the screen goes black on a phone or tablet, that's not a problem as long as the mandala image is set as the wallpaper or lock screen. As long as the mandala is set as the wallpaper and/or lock screen of your phone or tablet, its will be broadcast 24/7. even if the phone is turned off, it will continue to broadcast. As long as the mandala remains set as the wallpaper and/or lock screen, and as long as the battery remains charged, the energy will continue to be broadcast.
To turn the broadcast off on your phone, you must remove the mandala image from your wallpaper and/or lock screen and replace it with a different image.